The magnate, who these days spends more time talking about the "miracle of vaccines" than software, spoke in Senegal ahead of an address to the World Health Assembly in Geneva on Tuesday.

Gates' message: Technology is only as effective as the leaders delivering it.

Once you have vaccines invented and manufactured at low cost, and rich donors paying for them, "you have to have developing countries act to take the vaccines and get them out to all the kids".

"It is tragic when the last delivery piece holds it back," he said.

"You have some countries, northern Nigeria, or Chad or the DRC (Democratic Republic of Congo) where less than half the kids right now are getting the vaccines," he said, referring to polio immunisation in those countries.

"They are just not putting good people on it, not tracking their results, not even getting out to parts of the country. The message has to be clear. They have to see this as their top priority. It hasn't been."

The world is 99 per cent towards making polio the second disease to be wiped out, after smallpox. The paralysing disease, which once terrorised the developed world, exists now only in a few countries.

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Nigeria, the world's most troublesome polio spot, saw a 95 per cent cut in cases last year due to revitalised immunisation but a lack of support from government leaders, which Gates put down to the "distraction of the elections" in the country.

A spread to Chad and the DRC poses great risks for eradication efforts and the two countries carry 58 per cent of the total cases of polio this year.

"The key point is to help them get it right, many very, very poor countries do have 90 per cent coverage rates. It is not about trained doctors or anything complicated," Gates said.

When vaccines save lives it is "not because of hospitals or doctors or any of the expensive stuff. It doesn't require fancy equipment, simply keeping the vaccination cold and getting them out to all the kids."

In Africa, millions of children die from diarrhoeal diseases and pneumonia - the vaccines are too expensive. At the same time other deadly diseases such as malaria need new, low cost vaccines.

"A vaccine is truly unique, not only in reducing deaths but kids who have these diarrhoeal diseases end up never fully developing their brains. IQ studies done in Africa show huge deficits."

Gates has been rallying hard for donors to keep funding up as the Global Alliance for Vaccines and Immunisation seeks to fill a $US3.7 billion ($3.51 billion) funding gap at its June 13 pledging conference.

A new meningitis vaccine made specifically for Africa, with an Indian manufacturer bringing the price down to less than $0.50 a dose, is already saving thousands of lives in Burkina Faso.

In the first 12 weeks of this year there have been only two cases of meningitis in the country compared to 66 in the same period last year, Gates will announce.

If MenAfriVac is introduced throughout southern Africa, the resulting reduction in meningitis cases could free up to $US120 million ($113.7 million) from national budgets by 2010, according to the World Health Organsiation.

For Gates, when it comes to vaccines, a little goes a long way, and any initial money spent in getting the medicines out there has a massive impact in terms of lives and eventually millions saved in healthcare spending.

"As you improve health you are allowing people to develop and be educated."

The Microsoft founder has donated $US10 billion ($9.48 billion) of his sizeable fortune to bring vaccines to children in the next decade, calling on others to invest in vaccines as a cost-effective way to save lives and boost development.

His goal? Five or six new vaccines at an affordable price, and that every country has a system to deliver these vaccines to all children.

"For kids the malaria vaccine would be the next best thing. The funders need to stay behind that research despite the budget challenges they face," said Gates.